1. Field of the Invention
This invention relates to pumping apparatus for transporting fluids from a well formation to the earth's surface. More particularly, the invention pertains to a double-acting, reciprocating downhole pump.
2. Description of the Related Art
Many hydrocarbon wells are unable to produce at commercially viable levels without assistance in lifting formation fluids to the earth's surface. In some instances, high fluid viscosity inhibits fluid flow to the surface. More commonly, formation pressure is inadequate to drive fluids upward in the wellbore. In the case of deeper wells, extraordinary hydrostatic head acts downwardly against the formation, thereby inhibiting the unassisted flow of fluid to the surface.
A common approach for urging production fluids to the surface includes the use of a mechanically actuated, positive displacement pump. Mechanically actuated pumps are sometimes referred to as “sucker rod” pumps. The reason is that reciprocal movement of the pump necessary for positive displacement is induced through reciprocal movement of a string of sucker rods above the pump from the surface.
A sucker rod pumping installation consists of a positive displacement pump disposed within the lower portion of the production tubing. The installation includes a piston which is moved in linear translation within the tubing by means of steel or fiberglass rods. Linear movement of the sucker rods is imparted from the surface by a rocker-type structure. The rocker-type structure serves to alternately raise and lower the sucker rods, thereby imparting reciprocating movement to the piston within the pump downhole.
Certain difficulties are experienced in connection with the use of sucker rods. The primary problem is rooted in the fact that most wells are not truly straight, but tend to deviate in various directions en route to the zone of production. This is particularly true with respect to wells which are directionally drilled. In this instance, deviation is intentional. Deviations in the direction of a downhole well cause friction to occur between the sucker rod and the production tubing. This, in turn, causes wear on the sucker rod and the tubing, necessitating the costly replacement of one or both. Further, the friction between the sucker rod and the tubing wastes energy and requires the use of higher capacity motors at the surface.
In an attempt to overcome this problem, submersible electrical pumps have been developed. These pumps are installed into the well itself, typically at the lower end of the production tubing. State of the art submersible electrical pumps comprise a cylindrical assembly which resides at the base of the production string. The pump includes a rotary electric motor which turns turbines at a high horsepower. These turbines are placed below the producing zone of a well and act as fans for forcing production fluids upward through the production tubing.
Efforts have been made to develop a linear electric motor for use downhole. One example is U.S. Pat. No. 5,252,043, issued to Bolding, et al., entitled “Linear Motor-Pump Assembly and Method of Using Same.” Other examples include U.S. Pat. No. 4,687,054, issued in 1987 to Russell et al. entitled “Linear Electric Motor For Downhole Use,” and U.S. Pat. No. 5,620,048, issued in 1997, and entitled “Oil-Well Installation Fitted With A Bottom-Well Electric Pump.” In these examples, the pump includes a linear electric motor having a series of windings which act upon an armature. The pump is powered by a cable extending from the surface to the bottom of the well, and residing in the annular space between the tubing and the casing. The power supply generates a magnetic field within the coils which, in turn, imparts an oscillating force upon the armature. In the case of a linear electric motor, the armature would be translated in an up-and-down fashion within the well. The armature, in turn, imparts translational movement to a piston, or connector shaft, residing below the motor. The linear electric motor thus enables the piston of a positive displacement pump to reciprocate vertically, thereby enabling fluids to be lifted with each stroke of the piston.
Submersible pump assemblies which utilize a linear electric motor have not been introduced to the oil field in commercially significant quantities. Such pumps would suffer from several challenges, if employed. One such relates to the volume of fluids which can be lifted with each stroke. In this respect, the typical positive displacement pump will only capture fluids on either the upstroke or the downstroke, depending on its design. Most commonly, fluids are captured, or “gulped,” on the downstroke, with the captured volume of fluid flowing through a pump outlet at the top of the pump and then being lifted on the upstroke. Therefore, current positive displacement pumps are considered single acting, and not double-acting. Stated another way, fluid is only captured during a single phase of the stroke, and not during both phases of the stroke.
One obstacle encountered with the design of pumps pertains to hydrostatic balancing. In order to maximize efficiency of a motor apparatus for reciprocating a downhole pump, it is desirable that the pump be hydrostatically balanced. This means that the force required to move the pumping chamber on the upstroke is essentially the same as that required to move the pumping chamber back down on the down stroke. In the typical rocker-beam type lifting arrangement, the downhole pump is biased downward due to the action of hydrostatic head against the pump. Thus, the motor employed for lifting fluids via reciprocation of sucker rods requires that the motor have the capacity to lift a full column of fluid on the upstroke. The pump then simply falls back down on the downstroke in response to the weight of the sucker rods. Therefore, a linear electrical pump design which provides for hydrostatic balancing is desirable so that the force of the pump acting upward is used to displace fluids rather than to purely overcome the hydrostatic pressure differential.
In view of the above discussion, it is apparent that a more effective positive displacement pump is needed in order to transport formation fluids through the production tubing and to the earth's surface. In addition, a reciprocating pump is needed which is double-acting, that is, it is able to displace fluids both on the down stroke and on the upstroke. Further, a downhole pump is needed which permits the capture of a greater volume of fluids without a corresponding increase in velocity of the fluids through the pump. Further still, a linear pump is needed that is substantially hydrostatically balanced.